Method and apparatus for electrospray-augmented high field asymmetric ion mobility spectrometry

ABSTRACT

A field asymmetric ion mobility spectrometer apparatus and system including a sample preparation and introduction section, a head for delivery of ions from a sample, an ion filtering section, an output part, and an electronics part wherein the filter section includes surfaces defining a flow path, further including ion filter electrodes facing each other over the flow path that enables the flow of ions derived from the sample between the electrodes and wherein the electronics part applies controlling signals to the electrodes for generating a filter field for filtering the flow of ions in the flow path while being compensated to pass desired ion species out of the filter.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/070,904, filed on Mar. 3, 2005, which is a continuation of U.S.application Ser. No. 10/734,499, filed on Dec. 12, 2003, now U.S. Pat.No. 6,972,407, which is a continuation of U.S. application Ser. No.10/123,030, filed Apr. 12, 2002, now U.S. Pat. No. 6,650,004, which is acontinuation in part of: U.S. application Ser. No. 09/358,312, filedJul. 21, 1999, now U.S. Pat. No. 6,495,823; U.S. application Ser. No.09/439,543, filed Nov. 12, 1999, now U.S. Pat. No. 6,512,224; U.S.application Ser. No. 09/799,223, filed Mar. 5, 2001, now U.S. Pat. No.6,815,668; and U.S. application Ser. No. 10/040,974, filed Jan. 7, 2002,all of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to ion mobility spectrometry for gas and liquidsample preparation, filtering, and detection in a field asymmetricwaveform ion mobility spectrometer, with electrospray sample delivery,and using either internal or external detectors.

BACKGROUND

Electrospray mass spectrometry is a powerful analytical tool that hasbeen broadly applied to bio-molecular structure analysis (i.e.,Proteins, Peptides and DNA). See Electrospray Ionization MassSpectrometry Fundamentals, Instruments, and Applications, Richard B.Cole, John Wiley and Sons, 1997. This technique plays a central role inthe development of most pharmaceutical drugs and is being used toperform quantitative measurement of human exposure to carcinogens.Because of the size and potential revenues of the pharmaceutical market,there is interest in developing instrumentation based on, and technicalenhancements to, electrospray mass spectrometry.

In recent years there has been a general trend to minimize the amount ofsample required for analysis and micro-electrospray ionization(micro-ESI, micro-ES) and nanospray describe two of these approaches.These two methods share a lot in common, and they are often usedinterchangeably. Micro-ES is a miniaturized electrospray source with thesame system components as “conventional” electrospray. These include asource of pumped liquid flow containing the sample for analysis, a smalldiameter sharp hollow needle through which liquid is pumped, and asource of high voltage to generate the spray. Nanospray relies on theelectrostatic attraction of the liquid inside the needle towards anattractor counter-electrode to generate the flow rather than a pump.This characteristic makes nanospray very attractive as a means tominimize sample waste. Since electrospray, micro-ES, and nanospray areall species of a generic class referred to as electrospray they will beinterchangeably referred to as electrospray in this patent.

The nature of the electrospray ionization process makes samplepreparation a major consideration. The presence of solvent and buffersalts along with the sample significantly increases spectral complexityand degrades detection limits. The electrospray ionization processproduces an abundance of solvent ions that give an intense mass spectralbackground that can severely limit identification of many compounds attrace levels in solution. Even without the solvent ions to contend with,many applications require working with complex mixtures that necessitatesome degree of separation prior to mass analysis. See J. Lee, J. F.Kelly, I. Chernushevich, D. J. Harrison, and P. Thibalut “Separation andIdentification of Peptides from Gel-Isolated Membrane Proteins Using aMicrofabricated Device for Combined CapillaryElectrophoresis/Nanoelectrospray Mass Spectrometry,” Anal. Chem. 2000,72, 599-609. Better methods for elimination of unwanted solvent andseparation of sample ions from background are therefore needed.

Electrospray mass spectrometry (ES-MS) provides a powerful tool forstructure determination of peptides, proteins. This is important, asstructure to a large extent defines the function of the protein. Thestructural information about a protein is typically determined from itsamino acid sequence. To identify the sequence, the protein is usuallydigested by enzymes, and the peptide fragments are sequenced by tandemmass spectrometry. Another possible way to obtain the sequence is todigest the protein and measure the molecular weights of the peptidefragments. These are the input data for a computer program which digeststheoretically all the proteins being found in the data base and thetheoretical fragments are compared with the measured molecular weights.

Recently, it has been noticed that Ion Mobility Spectrometry can provideuseful information to an electrospray/mass-spectrometry measurement. IonMobility spectrometry is ordinarily an atmospheric pressure techniquewhich is highly sensitive to the shape and size of a molecule. Proteinidentification thorough the combination of an IMS and mass spectrometermay eliminate the need for protein digestion, simplifying samplepreparation.

Commercially available IMS systems are based on time-of-flight (TOF),i.e., they measure the time it takes ions to travel from a shutter-gateto a detector through an inert atmosphere (1 to 760 Torr.). The drifttime is dependent on the mobility of the ion (i.e., its size, mass andcharge) and is characteristic of the ion species detected. TOF-IMS is atechnique useful for the detection of many compounds includingnarcotics, explosives, and chemical warfare agents. See PCT ApplicationSerial No. PCT/CA99/00715 incorporated herein by this reference and U.S.Pat. No. 5,420,424 also incorporated herein by this reference. In ionmobility spectrometry, gas-phase ion mobility is determined using adrift tube with a constant low field strength electric field. Ions aregated into the drift tube and are subsequently separated based ondifferences in their drift velocity. The ion drift velocity under theseconditions is proportional to the electric field strength and the ionmobility, which is determined from experimentation, is independent ofthe applied field. Current spectrometers use conventionally machineddrift tubes (minimum size about 40 cm³) for ion identification.

In conventional time-of-flight ion mobility spectrometers (TOF-IMS) ionidentification is done in a low strength electric field (less than 1000V/cm) where the coefficient of mobility for each ion is essentiallyindependent of field strength [.W. McDaniel and Edward A. Mason, Themobility and diffusion of ions in gases, John Wiley & Sons, 1973].

At high electric fields, ion mobility becomes dependent upon the appliedelectric field strength and the ion drift velocity may no longer behavelinearly with field strength. This principle is utilized in the subjectof this disclosure.

The field asymmetric waveform ion mobility spectrometer (FAIMS, alsoknown as RF-IMS) utilizes these significantly higher electric fields,and identifies the ion species based on the difference in its mobilityin high and low strength electric fields.

The FAIMS spectrometer uses an ionization source, such as an ultraviolet photo-ionization lamp, to convert a gas sample into a mixture ofion species with each ion type corresponding to a particular chemical inthe gas sample. The ion species are then passed through an ion filterwhere particular electric fields are applied between electrodes toselect an ion type allowed to pass through the filter. Once through thefilter the ion type hits a detector electrode and produces an electricalsignal. To detect a mixture of ion species in the sample, the electricfields applied between the filter electrodes can be scanned over a rangeand a spectrum generated. The ion filtering is achieved through thecombination of two electric fields generated between the ion filterelectrodes, an asymmetric, periodic, radio frequency (RF) electricfield, and a dc compensation electric field. The asymmetric RF field hasa significant difference between its peak positive field strength andnegative field strength. The asymmetric RF field scatters the ions andcauses them to deflect to the ion filter electrodes where they areneutralized, while the compensation field prevents the scattering of aparticular ion allowing it to pass through to the detector. The ions arefiltered in instruments on the basis of the difference in the mobilityof the ion at high electric fields relative to its mobility at lowelectric fields. That is, the ions are separated due to the compounddependent behavior of their mobility at high electric fields relative totheir mobility at low electric fields.

The FAIMS approach is based on an observation of Mason and McDaniel [.W.McDaniel and Edward A. Mason, The mobility and diffusion of ions ingases, John Wiley & Sons, 1973] who found that the mobility of an ion isaffected by the applied electric field strength. Above an electric fieldto gas density ratio (E/N) of 40 Td (E>10,700 V/cm at atmosphericpressure) the mobility coefficient K(E) has a non-linear dependence onthe field. This dependence is believed to be specific for each ionspecies. Below are some examples from Mason and McDaniel [.W. McDanieland Edward A. Mason, The mobility and diffusion of ions in gases, JohnWiley & Sons, 1973]. The mobility for the cluster ion CO⁺CO increaseswith increasing field strength (FIG. 7-1-K-1 in reference [.W. McDanieland Edward A. Mason, The mobility and diffusion of ions in gases, JohnWiley & Sons, 1973]). For some molecular and atomic ions the coefficientof mobility can change in a more complex way. For example, for atomicions K⁺, the mobility coefficient in carbon monoxide gas increases withincreasing field by as much as 20%, but above E/N˜200 Td the coefficientstarts to decrease (FIG. 7-1-K-3 in reference [.W. McDaniel and EdwardA. Mason, The mobility and diffusion of ions in gases, John Wiley &Sons, 1973]). For some other ions for example N⁺, N₃ ⁺ and N₄ ⁺ themobility changes very little (FIG. 7-1-H-1/2 in reference [.W. McDanieland Edward A. Mason, The mobility and diffusion of ions in gases, JohnWiley & Sons, 1973]). FIG. 1A illustrates schematically three possibleion mobility dependencies on electric field. For simplicity we willassume that the low field value of the mobility K(E_(min)) in a weakelectric field (E approximately 10²-10³ V/cm) is the same for all threeion types. However, at E_(max) the value of the mobility coefficientK(E_(max)) is different for each ion type.

The field dependence of the mobility coefficient K(E) can be representedby a series expansion of even powers of E/N [18]K(E)=K(0)[1+α₁(E/N)²+α₂(E/N)⁴+ . . . ]  (1)where K(0) is the coefficient of mobility of the ion in a weak electricfield, and α₁, α₂ are coefficients of the expansion. This equation canbe simplified by using an effective α(E) as shown in equation 2 [T. W.Carr, Plasma Chromatography, Plenum Press, New York and London, 1984],K(E)≈K(0)[1+α(E)].  (2)According to this expression when α(E)>0 the mobility coefficient K(E)increases with field strength, when α(E)˜0 the mobility K(E) does notchange, and when α(E)<0 then K(E) decreases with increasing fieldstrength. An expression for the field dependent mobility coefficient canalso be derived from momentum and energy balance considerations. Wherethe energy of the ion ε=3/2 kT_(eff) can be expressed as a function ofits effective temperature [18-20].

$\begin{matrix}{{K(E)} = {\frac{\nu}{E} = {\frac{q}{N}\left( \frac{1}{3\mu\;{kT}_{eff}} \right)^{1/2}{\frac{1}{\Omega\left( T_{eff} \right)}.}}}} & (3)\end{matrix}$The case where α(E)<0 can be explained based on the model presented inequation 3, if one assumes the value of the ion neutral cross-sectionΩ(T_(eff)) does not change significantly for rigid-sphere interactions[T. W. Carr, Plasma Chromatography, Plenum Press, New York and London,1984, E. A. Mason and E. W. McDaniel, Transport Properties of Ions inGases, Wiley, New York, 1988] and the reduced mass μ is constant. Underthese conditions one finds that the mobility K(E) will decrease if theeffective temperature, or energy, of the ion increases. Physically thiseffect has a simple explanation. When the electric field strength isincreased the ions are driven harder through the neutral gas. Thisincreases the ion neutral collision frequency, which leads to a reducedaverage ion velocity and a reduced ion mobility coefficient.

The rigid-sphere model however, does not explain the experimentalresults which show that with certain ions the mobility increases withincreasing electric field (α(E)>0). One of the possible explanations forthe increased mobility at elevated values of E/N is offered when oneallows for ion de-clustering at high field strengths to occur. Ions inambient conditions in a weak electric field generally do not exist in afree state. They are usually in cluster form (for example, MH⁺(H₂O)_(n))with n polar molecules such as water attached. As the electric fieldstrength is increased the kinetic energy and consequently the effectivetemperature (T_(eff)) of the ion increases due to the energy impartedbetween collisions. This can lead to a reduction in the level of ionclustering (reduction in n) resulting in a smaller ion cross-sectionΩ(T_(eff)) and a smaller reduced mass μ for the ion. According toequation 3 then, if do to de-clustering the cross-section and reducedmass decrease in a sufficient manner to offset the increase in T_(eff)the case where α(E)>0 can be explained.

The third case when α(E)˜0 can be explained by a decrease in ion crosssection due to de-clustering which is offset by an increase in theeffective temperature of the ion. This results in no net change to themobility coefficient of the ion.

The mechanism of operation of the FAIMS for ion filtering is describedin the following. Consider three kinds of ions with different mobilitycoefficient dependencies on electric field (i.e., α(E)>0, α(E)<0,α(E)˜0) which are formed, due to local ionization of neutral molecules,at the same location in a narrow gap between two electrodes, as shown onFIG. 1B. A stream of carrier gas transports these ions longitudinallydown the drift tube between the gap. If an asymmetric RF electric fieldis then applied to the electrodes the ions will oscillate in aperpendicular direction to the carrier gas flow, in response to the RFelectric field, while moving down the drift tube with the carrier gas. Asimplified asymmetric RF electric field waveform (FIG. 1C) with maximumfield strength |E_(max)|>10,000 V/cm and minimum field strength|E_(min)|<<|E_(max)| is used here to illustrate the operation principleof the RF-IMS. The asymmetric RF waveform is designed such that the timeaverage electric field is zero and|E _(max) |t ₁ =|E _(min) |t ₂=β.  (1)t₁ is the portion of the period where the high field is applied and t₂is the time the low field is applied. β is a constant corresponding tothe area under-the-curve in the high field and low field portions of theperiod. The ion velocities in the y-direction are given byV _(y) =K(E)E(t).  (2)Here K is the coefficient of ion mobility for the ion species and E isthe electric field intensity, in this case entirely in the y-direction.If the amplitude of the positive polarity RF voltage pulse (during t₁)produces an electric field of strength greater than 10,000 V/cm then thevelocity towards the top electrodeV _(up) =K _(up) |E _(max)|  (3)will differ for each of the ion species (FIG. 1B) since, as shown inFIG. 1A, the coefficient of mobility K_(up) for each ion at the highfield condition is different. The ions with α(E)>0 will move faster andions with α(E)<0 will have the smallest velocity, therefore, the slopeof each ion's trajectory will also differ. In the next portion of theperiod (t₂), once the polarity of the RF field has switched, all threeion types will begin moving with the same velocityV _(down) =K(E _(min))|E _(min)|  (4)down towards the bottom plate. In this low field strength condition (seeFIG. 1A) all three ion types will have the same mobility coefficientK_(down). Therefore, all three ion trajectories will have the same slopein this portion of the period (FIG. 1B).

The ion displacement from its initial position in the y-direction is theion velocity in the y-direction V_(y) multiplied by the length of timeΔt the field is appliedΔy=V_(y)Δt.  (5)In one period of the applied RF field the ion moves in both the positiveand negative y-directions. By substituting equation 2 into equation 5the average displacement of the ion over one period of the RF field canbe written asΔy _(RF) =K _(up) |E _(max) |t ₁ −K _(down) |E _(min) |t ₂.  (6)Using equation 1 this expression can be re-written asΔy _(RF)=β(K _(up) −K _(down))=βΔK.  (7)Since β is a constant determined by the applied RF field, they-displacement of the ion per period of the RF field T=t₁+t₂ depends onthe change in mobility of the ion between its high and low fieldconditions. Assuming the carrier gas only transports the ion in thez-direction. The total ion displacement Y (in the y-direction) from itsinitial position (due to the electric field) during the ions residencetime t_(res) between the ion filter plates can be expressed as

$\begin{matrix}{Y = {{\frac{\Delta\; y_{RF}}{\left( {t_{1} + t_{2}} \right)}t_{res}} = {\frac{{\beta\Delta}\; K}{T}t_{res}}}} & (8)\end{matrix}$The average ion residence time inside the ion filter region is given inequation 9. A is the cross-section area of the filter region, L is thelength of the ion filter electrodes, V is the volume of the ion filterregion V=AL, and Q is the volume flow rate of the carrier gas.

$\begin{matrix}{t_{res} = {\frac{AL}{Q} = {\frac{V}{Q}.}}} & (9)\end{matrix}$Substituting equation 9 into equation 8, noting from equation 1 thatβ=|E_(max)|t₁ and defining the duty cycle of the RF pulses as D=t₁/T.The equation for displacement of the ion species, equation 8, can bere-written as

$\begin{matrix}{Y = \frac{\Delta\;{KE}_{\max}{VD}}{Q}} & (10)\end{matrix}$where Y is now the total displacement of the ion in the y-directionbased on the average ion residence time in the ion filter region. Fromequation 10 it is evident that the vertical displacement of the ions inthe gap are proportional to the difference in coefficient of mobilitybetween the low and high field strength conditions. Different species ofions with different ΔK values will displace to different values of Y fora given t_(res). All the other parameters including the value of themaximum electric field, the volume of the ion filter region, the dutycycle and the flow rate, to first order are essentially the same for allion species.

When a low strength DC field (|E_(c)|<|E_(min)|<<|E_(max)|) is appliedin addition to the RF field, in a direction opposite to the averageRF-induced (y-directed) motion of the ion, the trajectory of aparticular ion species can be “straightened”, see FIGS. 1D(1), 1D(2),1D(3). This allows the ions of a particular species to pass unhinderedbetween the ion filter electrodes while ions of all other species aredeflected into the filter electrodes. The DC voltage that “tunes” thefilter and produces a field which compensates for the RF-induced motionis characteristic of the ion species and is called the compensationvoltage. A complete spectrum for the ions in the gas sample can beobtained by ramping or sweeping the DC compensation voltage applied tothe filter. The ion current versus the value of the sweeping voltageforms the RF-IMS spectra. If instead of sweeping the voltage applied toone of the ion filter electrodes, a fixed DC voltage (compensationvoltage) is applied, the spectrometer will work as continuous ion filterallowing only one type of ion through.

In PCT Application Serial No. PCT/CA99/00715, an electrospray ionizationchamber or electrospray source is used to create ions which areultimately transported to an analytical region which is subject to botha high frequency voltage asymmetric waveform and a DC offset voltage.

It is therefore an object of the present invention to provide method andapparatus for improved detection of compounds using field asymmetricwaveform ion mobility.

SUMMARY

Objects of the invention are achieved in practice of field asymmetricion mobility spectrometers and novel improvements, particularly in threeareas: 1) sample preparation and introduction, 2) ion filtering, and 3)output and signal collection.

Embodiments of the invention feature combinations of various aspects,including use of a FAIMS ion filter to filter ions where control ofwhich ions are filtered is achieved by control of a variable DCcompensation signal in addition to a high field asymmetric waveformradio frequency signal or use of a FAIMS filter where the control ofwhich ions are filtered is achieved by varying the wavelength,frequency, amplitude, period, duty cycle or the like of the high fieldasymmetric waveform radio frequency signal; use of a planar FAIMS filterwhich uses insulating substrates to very accurately control the gapbetween the ion filter electrodes and ensure the ion filter electrodesare parallel, this allows very reproducible fields to be obtained whichresults in a higher resolution spectrometer; use of a planar FAIMSfilter where the insulating spacers overlap the edges of the filterelectrodes, which results in a higher resolution FAIMS with moreaccurate identification of compounds since all the sample is forced topass between the ion filters and no ions can bypass the filterelectrodes and still reach the detector electrode.

In use with a spray source, such as electrospray, where desolvation ofthe ions is very important in order to obtain reliable, reproduciblespectra, desolvation is achieved. Desolvation electrodes may be includedto assist in desolvation, where enhanced desolvation is achieved byapplying symmetric RF signals to the desolvation electrodes. The RFsignals provide energy to the ions which raises their effectivetemperature and helps to enhance the desolvation process.

Desolvation electrodes can also be used to control the level of ionclustering in gas samples from electrospray and from other thanelectrospray sources. Control of ion clustering can permit morerepeatable measurements and also can provide additional information onthe ions being detected.

A novel embodiment of the invention relates to the sample preparationsection. This embodiment incorporates the use of an electrospray headand the use of an attraction electrode which is separated from the ionfilter electrodes. The advantage of separating the attraction electrodefrom the ion filter electrodes is that this allows freedom in applying adifferent potential to the attraction electrode relative to the ionfilter electrodes, and this allows optimization of the electrosprayconditions and ion introduction conditions into the FAIMS. Thisseparation of attraction electrode from the ion filter electrodes canalso be realized in cylindrical FAIMS configurations.

Additionally, guiding electrodes can be provided and allow furtheroptimization of ion injection into the ion filter. In a furtherembodiment of the invention the electrospray assembly can be attached toone of the substrates of the FAIMS and guiding electrodes are used toguide the ions into the ionization region. The guiding electrodes can bea freestanding structure attached or connected to or near one of thesubstrates of the FAIMS. The assembly can have a counter gas flow toenhance desolvation.

The invention also features the realization of the concept that atime-of-flight measurement can be combined with a FAIMS approach usingelectrospray to provide improved identification of the ion speciesthrough the additional information provided by the time-of-flightmeasurement. The time it takes the ion to travel from the orifice of theFAIMS to the detector can be measured. This can be achieved through theindependent control of the attraction and guiding electrode potentials.For example, initially the attraction electrode potential is adjusted sothat no ions make it into the drift region, but rather are collected atthe guiding electrodes. Then the attraction electrode is pulsed so thatsome ions can make it into the ionization region and into the ionfilter. Now the time it takes the ions to travel from the ionizationregion to the detector can be measured, and this provides additionaldiscriminating information on the identity of the ion.

A novel aspect of the invention is the concept of formation ofelectrodes on an insulating or insulated substrate where the insulatingsubstrate can form a housing. This approach provides significantadvantages in simplification of device construction. It allows low cost,mass producible processes to be used such as micromachining andmultichip modules which can result in low cost, miniature sensors.

In the output section, the embodiments of the FAIMS proposed are thefirst to have output sections with the ability to detect multiple ionspecies simultaneously such as a positively and negatively charged ion.

Since sample analysis in the FAIMS is generally performed in the gasphase, liquid samples require conversion from the liquid to the gasphase. In a preferred embodiment, the electrospray method (which wedefine as encompassing “conventional”, micro and/or nanospray) is usedto convert a liquid sample into gas phase ions. Preferably the ionsstreaming out of the electrospray tip are submitted to a planar FAIMSdevice. In a preferred practice of the invention, all the functions ofsample preparation, ionization, filtering and detection are performed ona single “chip”.

In another embodiment, the electrospray-FAIMS is applied as a filter toa mass spectrometer. The FAIMS coupled to the mass spectrometer providesenhanced resolution, better detection limits, ability to extract shapeand structure information of the molecules being analyzed, molecules caninclude bio-molecules such as proteins and peptides. The FAIMS techniqueis based on ion mobility, where ion filtering and identification ishighly dependent on the size and shape of the ion. This information isof great interest in genomics and proteomics research (i.e.,pharmaceutical industry) since the shape of a protein to a large extentdetermines its functionality and therefore FAIMS filtering can beapplied as a low cost high volume method of protein characterization. Aparticular embodiment includes a disposable FAIMS filter chip which isplugged into a carrier mounted on the inlet of a mass spectrometer. TheFAIMS-electrospray device can also provide structural (conformation)information about the molecule being analyzed and sequence informationnot obtainable simply with electrospray-mass spectrometry. In additionthe FAIMS allows discrimination between isomers (molecules with theidentical mass but which differ in their shape) which cannot beidentified using electrospray-mass spectrometry alone.

In a particular embodiment, the electrospray-FAIMS forms a filter anddetection system in a single housing. The electrospray-FAIMSconfiguration of the present invention can be used as a standalonedetector for liquid sample analysis or as the front end to a massspectrometer. The present invention also has application to other liquidseparation techniques such as liquid chromatography, high pressureliquid chromatography, and capillary electrophoresis. A preferredembodiment of the invention includes a planar FAIMS apparatus where inone embodiment the device is integrated with an electrospray ionizingsource on a common housing or substrate and is coupled to a massspectrometer. Alternative practices of the invention may includecylindrical or coaxial FAIMS devices.

Embodiments of the invention enable filtering of molecules after theyhave been ejected from a source, such as from an electrospray tip or acapillary electrophoresis outlet, and have been ionized prior tofiltering via a FAIMS filter, and detected via an internal detector orvia a mass spectrometer or other detector. In one practice of theinvention, micromachining (MEMS) processing enables integration of anelectrospray tip with a FAIMS filter into a simple device and results ina precise yet compact analytical system for accurate, highly repeatable,liquid sample evaluation. In another practice of the invention,portable, miniature, low cost, bio-sensors for biological agentdetection which use an integrated electrospray-FAIMS chip are possible;preferably they are prepared using micromachining fabricationtechniques. In one embodiment an atmospheric pressure chemicalionization (APCI) device is achieved with a FAIMS filter used as aprefilter to a mass spectrometer.

Prior to the present invention, conventional machining led to high costof fabrication and poor reproducibility from FAIMS device to FAIMSdevice. Furthermore, prior art cylindrical FAIMS geometry either limitscollection efficiency when interfacing to a mass spectrometer, orpermits both sample neutrals and sample ions to enter the massspectrometer, resulting in more complex spectra. Advantageously inpractice of the invention, ion filtering is performed after sampleionization, therefore buffer salt and solvent ions, which are invariablygenerated in the electrospray process, are separated from thebio-molecules of interest. This provides significantly simpler massspectra and improves the detection limits and identification of thebio-molecules.

Combination of electrospray with a new FAIMS filter device enablesanalytical detection devices with greatly enhanced sensitivity andresolution. In some cases the ability is provided to resolve compoundsthat could not be identified without the FAIMS present. Combination ofelectrospray with a prior art FAIMS filter devices raises issues ofsample to sample contamination when running low concentration samplesthrough the device for high throughput low cost sample analysis, butthese are overcome in practice of the present invention.

The new FAIMS of the invention is a low cost, a volume manufacturable,small and compact, spectrometer based on differential ion mobility. Thepresent invention, particularly configured using high volumemanufacturing techniques, such as MEMS fabrication techniques whichincludes ceramic packaging, PC board manufacturing techniques or plasticprocessing, offers several additional advantages over prior devices. Thevolume manufacture techniques result in low cost devices that can bemade disposable, thus avoiding the problem of sample crosscontamination. These chips will be available to any laboratory using amass spectrometer for biological molecule identification as a FAIMSinterface filter. Such a filter includes the FAIMS interface chip whichcan plug into an interface fixture which contains, filteringelectronics. The electrospray tip or electrophoresis chips can beintegrated with (fabricated as part of) the FAIMS chip. The MEMSapproach is not required but is preferred and renders high reliabilityand repeatability in volume manufactured FAIMS chips; this lowers theircost and enables disposable devices. This disposability avoidscontamination from one sample to the next, which is invaluable for testsperformed subject to, for and/or by regulatory agencies like the EPA andFDA where contamination is a concern.

In one embodiment of the present invention, a planar MEMS FAIMS chip wasfabricated in which ions are focused into a mass spectrometer andcollection efficiency is close to 100%. In this embodiment, no ioninjection is required into the FAIMS ion filter region. The device ismicromachined on a planar surface. This enables easy integration withonboard heaters to minimize ion clustering. It can be easily integratedwith micromachined or conventional electrospray tips and/ormicromachined electrophoresis chips. This is a simplified design withreduced fabrication requirements, and can be configured to use only asingle gas flow channel.

Micromachining provides for excellent reproducibility in the manufactureand performance of the filters. This is critical so that test resultsare consistent from one device to the next and from one laboratory tothe next. Micromachining enables new configurations of FAIMS filterchips which cannot be made any other way. These new configurations aresimpler and more efficient at delivering ions to the mass spectrometerand filtering unwanted ions.

A MEMS FAIMS drift tube has been successfully fabricated andcharacterized. High spectrometer sensitivity and ability to resolvechemicals not separated in conventional TOF-IMS has been demonstrated.The MEMS FAIMS enables the realization of miniature, low cost, highsensitivity, high reliability chemical detectors. The FAIMS spectrometerof the invention has also been demonstrated as a pre-filter to a massspectrometer. The new FAIMS/MS combination allows better resolution ofcomplex mixtures.

Portable, miniature, low cost, bio-sensors for biological agentdetection which use an integrated electrospray-FAIMS chip are possibleusing microfabrication methods such as micromachining s because of thesize reduction and cost reductions enabled by this technology andenabled manufacture. These instruments will have many uses, includingavailing high quality bio-analysis in the field. For example, a personsuspected of being exposed to a bio-agent will supply a drop of blood tothe instrument. The blood will be mixed with a buffer solution,processed, and introduced via the electrospray nozzle into the FAIMSwhere the ions will be analyzed. If a particular bio-molecule isdetected an alarm will be set off. As well, micromachining enables newconfigurations of FAIMS filter chips which are not otherwise available.For example, the planar FAIMS disclosed in this patent. These newconfigurations are simpler and more efficient at delivering ions to themass spectrometer and filtering unwanted ions. In a preferredembodiment, the Electrospray and FAIMS form an atmospheric pressurechemical ionization (APCI) prefilter and analyte filter and detectionsystem in a single housing. The new FAIMS-APCI provides high performanceand low cost, volume manufacturable, small and compact, ion mobilityspectrometer.

In an embodiment of the invention, a breakthrough can be attributed toproviding a multi-use housing/substrate/packaging that simplifiesformation of the component parts and resulting assembly. Additionalfeatures include the possibility to use the substrate as a physicalplatform to build the filter upon and to give structure to the wholedevice, to use the substrate as an insulated platform or enclosure thatdefines the flow path through the device, and/or use the substrate toprovide an isolating structure that improves performance. A spacer canbe incorporated into the device, which provides both a definingstructure and also the possibility of a pair of silicon electrodes forfurther biasing control. Multiple electrode formations and a functionalspacer arrangement can be utilized which improve performance andcapability. Filtering employs the FAIMS asymmetric periodic voltageapplied to the filters along with a control component, and thiscomponent can be a bias signal or voltage or may be supplied simplyotherwise, such as by control of the duty cycle of the same asymmetricsignal and which removes the need for the DC compensation circuit. Thiscompact arrangement enables inclusion of a heater for purging ions, andmay even include use of the existing electrodes, such as filter ordetector electrodes, for heating/temperature control.

Embodiments of the invention include a field asymmetric ion mobilityspectrometer apparatus and some preferred embodiments include a samplepreparation and introduction section, ion filtering section, and anoutput section and a control section, the filter comprising a FAIMS ionfilter for filtering ions. Embodiments of the invention may variouslyinclude: planar FAIMS filter which uses insulating substrates to veryaccurately control the gap between the ion filter electrodes and ensurethe ion filter electrodes are parallel, this allows very reproduciblefields to be obtained which results in a higher resolution spectrometer;a FAIMS filter where the insulating spacers overlap the edges of thefilter electrodes. This results in a higher resolution FAIMS with moreaccurate identification of compounds since all the sample is forced topass between the ion filters and no ions can bypass the filterelectrodes and still reach the detector electrode; an electrospray headand providing desolvation of the ions via desolvation electrodes;enhanced desolvation is achieved by applying symmetric RF signals to thedesolvation electrodes; the RF signals provide energy to the ions whichraises their effective temperature and helps to enhance the desolvationprocess; wherein desolvation electrodes for control of the level of ionclustering in gas samples, for more repeatable measurements andproviding additional information on the ions being detected, areprovided in a practice of the invention.

A novel aspect may include the concept of formation of electrodes on aninsulating or insulated substrate where the insulating substrate canform a housing, and providing significant advantages in simplificationof device construction, with low cost, mass producible processes to beused such as micromachining and manufacture of multichip modules whichcan result in low cost, miniature sensors using FAIMS architecture;within the output section the ability to detect multiple ion speciessimultaneously such as a positively and negatively charged ion; furtherincorporating the use of an electrospray head and the use of anattraction electrode which is separated from the ion filter electrodesto permit applying a different potential to the attraction electroderelative to the ion filter electrode(s), and this allows optimization ofthe electrospray conditions and ion introduction conditions into theFAIMS device. The FAIMS device may be cylindrical-type, planar-type, orotherwise. Guiding electrodes can allow further optimization of ioninjection into the ion filter.

It is also possible to form a time-of-flight measurement device combinedwith the FAIMS device using electrospray to provide improvedidentification of the ion species through the additional informationprovided by the time-of-flight measurement; the time it takes the ion totravel from the orifice of the FAIMS to the detector can be measured,which can be achieved through the independent control of the attractionand guiding electrode potentials; wherein the electrospray assembly canbe attached to one of the substrates of the FAIMS and guiding electrodesare used to guide the ions into the ionization region; a counter gasflow enhances desolvation; wherein the guiding electrodes can be afreestanding structure attached or connected to or near one of thesubstrates of the FAIMS; wherein control of which ions are filtered isachieved by control of a variable DC compensation signal in addition toa high field asymmetric waveform radio frequency signal, or control ofwhich ions are filtered is achieved by varying an aspect of the fieldsuch as the duty cycle, amplitude or frequency of the high fieldasymmetric waveform radio frequency signal, among others.

All the functions of sample preparation, ionization, filtering anddetection can be performed on a single chip or workpiece of theinvention; wherein an electrospray-FAIMS is applied as a filter to amass spectrometer; a chip carrier and a disposable FAIMS filter chipwhich is plugged into the carrier, the carrier enable for mounting onthe inlet of a mass spectrometer; wherein an electrospray-FAIMS forms afilter and detection system in a single housing; whereinelectrospray-FAIMS configuration of the present invention can be used asa standalone detector for liquid sample analysis or as the front end toa mass spectrometer; wherein present invention also has application toother liquid separation techniques such as liquid chromatography, highpressure liquid chromatography, and capillary electrophoresis; wherein apreferred embodiment of the invention includes a FAIMS apparatus wherein one embodiment the FAIMS device is integrated with an electrosprayionizing source on a common housing or substrate and is coupled to amass spectrometer; wherein embodiments of the invention enable filteringof molecules after they have been ejected from a source, such as from anelectrospray tip or a capillary electrophoresis outlet, and have beenionized prior to filtering via a FAIMS filter, and detected via aninternal detector, or via a mass spectrometer or other detector, and ina practice of the invention, micromachining (MEMS) processing enablesintegration of an electrospray tip with a planar field asymmetricwaveform ion mobility spectrometer filter into a simple unit/device andresults in a precise yet compact analytical system for accurate, highlyrepeatable, liquid sample evaluation, or in another practice of theinvention, portable, miniature, low cost, bio-sensors for biologicalagent detection which use an integrated electrospray-FAIMS chip arepossible, possibly prepared using micromachining fabrication techniques;wherein the FAIMS part is planar and forms an atmospheric pressurechemical ionization (APCI) prefilter to a mass spectrometer; wherein ionfiltering is performed after sample ionization, therefore buffer saltand solvent ions, which are invariably generated in the electrosprayprocess, are separated from the bio-molecules of interest and thisprovides significantly simpler mass spectra and improves the detectionlimits and identification of the bio-molecules; wherein combination ofelectrospray with a FAIMS filter device enables analytical detectiondevices with greatly enhanced sensitivity and resolution and wherein insome cases the ability is provided to resolve compounds that could notbe identified without the FAIMS present, and wherein combination ofelectrospray with a prior art FAIMS filter devices raises issues ofsample to sample contamination when running low concentration samplesthrough the device, high throughput low cost sample analysis, but theseare overcome in practice of the present invention.

A mass spectrometer is directly coupled to the exhaust port at the endof the drift tube, wherein a baffle may be placed to regulate thevelocity of waste gas flow stream relative to the velocity of drift gasflow stream, in a practice of the invention. Various sample preparationsections can be used, whether simply a port to draw in ambient airsamples, or electrospray, gas chromatograph, liquid chromatograph, orthe like. A split gas flow may be used to prevent clustering and allowsbetter identification of ion species.

The relationship between the amount of monomer and cluster ions for agiven ion species is dependent on the concentration of sample and theparticular experimental conditions (e.g., moisture, temperature, flowrate, intensity of RF-electric field). In a practice of the invention,both monomer and cluster states are detected to provide usefulinformation for chemical identification. In one example, a planar twochannel FAIMS is used to achieve this result, wherein a curtain gas isapplied to sample neutrals and they are prevented from entering thesecond channel “II” and ions in the monomer state can be investigated.In another embodiment, curtain gasses may flow in the same direction andexhaust at an orifice or in opposite directions while guiding electrodesare included to guide the ions into the second channel “II” and anattraction electrode is also used to attract ions into channel “II”,such that when the curtain gas is turned off ions in the cluster statemay be observed since sample neutrals and sample ions may now be drawninto channel “II” using a pump. The output section may be connected to amass spectrometer.

A method of the invention includes coupling a FAIMS device to a massspectrometer, for providing enhanced resolution, better detectionlimits, ability to extract shape and structure information of themolecules being analyzed, molecules can include bio-molecules such asproteins and peptides, the FAIMS technique being based on ion mobility,where ion filtering and identification is highly dependent on the sizeand shape of the ion, the FAIMS-electrospray device providing structural(conformation) information about the molecule being analyzed andsequence information not obtainable simply with electrospray-massspectrometry and also allowing discrimination between isomers (moleculeswith the identical mass but which differ in their shape) which cannot beidentified using electrospray-mass spectrometry alone.

Embodiments of the invention feature a multi-functional use of the FAIMSsubstrates. The substrates are platforms (or a physical supportstructures) for the precise definition and location of the componentparts or sections of the device. The substrates form a housing,enclosing the flow path with the filter and perhaps the detector, aswell as other components. This multi-functional design reduces partscount while also precisely locating the component parts so that qualityand consistency in volume manufacture can be achieved. The smallerdevice also has unexpected performance improvements, perhaps because ofthe smaller drift tube and perhaps also because substrates also performan electronic isolation function. By being insulating or an insulator(e.g., glass, ceramic, plastic), the substrates can be a direct platformfor formation of components, such as electrodes, with improvedperformance characteristics.

Embodiment of the invention may be cylindrical or planar, or the like,.In disclosed embodiments, use of substrates as a support/housing doesnot preclude yet other “housing” parts or other structures to be builtaround the device. For example, it might be desirable to put a humiditybarrier over the device. As well, additional components, like batteries,can be mounted to the outside of the substrate/housing, e.g., in abattery enclosure. Nevertheless, embodiments of the presently claimedinvention can provide a substrate insulation function, support function,multi-functional housing function, as well as other functions.

The insulative or insulated substrate/flow path invention achievesexcellent performance in a simplified structure. The use of anelectrically insulated flow path in a FAIMS device enables the appliedasymmetric periodic voltage to be isolated from the output part (e.g.,from the electrodes of the detector), where detection takes place. Thisreduction is accomplished because the insulated substrates provideinsulated territory between the filter and detector in the flow path,and this spacing in turn advantageously separates the filter's fieldfrom the detector. The less noisy detection environment means a moresensitive FAIMS device.

Moreover, by forming the electrodes on an insulative substrate, the ionfilter electrodes and detector electrodes can be positioned closertogether which unexpectedly enhances ion collection efficiency andfavorably reduces the device's mass that needs to be regulated, heatedand controlled. This also reduces power requirements. Furthermore, useof small electrodes reduces capacitance which in turn reduces powerconsumption. As well, tightly spaced electrodes lends itself to a massproduction process, since the insulating surfaces of the substrates area perfect platform for the forming of such electrodes.

Embodiments of the claimed invention result in FAIMS devices thatachieve high resolution, fast operation and high sensitivity, yet with alow parts count and in configurations that can be cost-effectivelymanufactured and assembled in high volume. Quite remarkably, packagingis very compact for such a capable FAIMS device, with sensitivity in therange of parts per billion or trillion. In addition, the reduced realestate of this smaller device leads to reduced power requirements,whether in sensing ions or in heating the device surfaces, and canenable use of a smaller battery. The benefits of the simplified andcompact FAIMS spectrometer according to the invention requires typicallyas little as one second (and even less) to produce a complete spectrumfor a given sample. No FAIMS system has ever been taught or disclosed inthe prior art that can achieve such beneficial results.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1A shows the mobility dependence on electric field for threedifferent ion species.

FIG. 1B shows the trajectories of ions in the gap between the upper andlower parallel plate electrodes of the ion filter, under thesimultaneous influence of the carrier gas flow and an asymmetric radiofrequency electric field waveform.

FIG. 1C shows a simplified asymmetric RF electric field waveform usedfor ion filtering.

FIG. 1D1 shows compensation voltage applied to cancel out displacementproduced by RF-field.

FIG. 1D2 shows the trajectory of ion from initial position with only theRF field applied and the trajectory of ion with only the compensationfield applied.

FIG. 1D3 shows the trajectory of ion with both RF and compensationfields applied.

FIG. 2 is a schematic of a chemical sensor system in practice of theinvention.

FIG. 3A shows a chemical sensor system with liquid sample preparationsection including an electrospray in practice of the invention.

FIG. 3B shows a chemical sensor system with liquid sample preparationsection including an electrospray in practice of the invention.

FIG. 3B1 shows a machined electrospray head in practice of theinvention.

FIG. 3C shows a serpentine electrode in practice of the invention.

FIG. 3D shows the substrates forming a housing in practice of theinvention.

FIG. 4A shows a FAIMS spectrometer with spaced insulated substrates inpractice of the invention.

FIG. 4B shows an alternative structural electrode embodiment in practiceof the invention.

FIG. 4C shows side cross-sectional view of filter with insulatingspacers overlapping edges of electrodes in practice of the invention.

FIG. 4D shows an electrospray head with a sample reservoir feeding aseparation channel leading to a spray tip in practice of the invention.

FIG. 5A shows symmetric AC radio frequency field for ion desolvation inpractice of the invention.

FIG. 5B shows the desolvation region integrated into a FAIMS device inpractice of the invention.

FIG. 6 shows a prior art cylindrical FAIMS connected to a massspectrometer.

FIGS. 7A and 7B show improved cylindrical FAIMS devices in practice ofthe invention.

FIG. 8 shows an electrospray mounting tower in practice of theinvention.

FIG. 9A shows an electrospray head cooperating with guiding electrodesin practice of the invention.

FIG. 9B shows an electrospray head cooperating with guiding electrodesin practice of the invention.

FIG. 10A shows the control system in practice of the invention.

FIG. 10B shows control signals in practice of the invention.

FIG. 11A shows a chip receptacle in practice of the invention.

FIG. 11B shows a chip receptacle interfaced with a mass spectrometer inpractice of the invention.

FIGS. 12A and 12B show planar FAIMS in practice of the invention.

FIGS. 12C and 12D show prior art cylindrical FAIMS devices.

FIGS. 13A and 13B show an electrospray tip inserted within the ionregion, either from above through orifice in upper substrate or from theside in practice of the invention.

FIGS. 14A and 14B show longitudinal electric field driven embodiments inpractice of the invention.

FIGS. 15A and 15B show split gas flow embodiments in practice of theinvention.

FIG. 16 shows a dual channel embodiment in practice of the invention.

FIG. 17 shows dependence of Ketones on compensation voltage fordifferent ionization sources in practice of the invention.

FIG. 18 shows a dual channel embodiment in practice of the invention.

FIG. 19 shows detection spectra in practice of the invention.

ILLUSTRATIVE DESCRIPTION

A description of preferred embodiments of the invention follows. Thepresent invention provides method and apparatus for analysis ofcompounds in a liquid sample, preferably enabled by high fieldasymmetric waveform ion mobility spectrometry.

In an illustrative embodiment of the present invention shown in FIG. 2,a chemical sensor system 10 includes a sample preparation section 10A, afilter section 10B, and an output section 10C. In practice, a liquidsample S is ionized in sample preparation section 10A, the created ionsthen being passed to and filtered in filter section 10B, and then ionspassing through the filter section are delivered to output section 10Cfor detection. The liquid sample preparation section 10A, filter section10B, and output section 10C operate under control and direction ofcontroller section 10D. Preferably controller section 10D controls boththe operation of system 10 and appraises and reports detection data D.

In a preferred embodiment of the invention, the liquid samplepreparation section 10A includes an electrospray head, which receives,conditions, and ionizes liquid sample S. This is transported to apreferred planar high field asymmetric ion mobility spectrometer(PFAIMS) filter in section 10B, the latter filtering the delivered ionsand passing ion species of interest to output section 10C. In variousembodiments of the invention, function in output section 10C may includeimmediate detection of ion species or transfer of ions to anothercomponent such as a mass spectrometer (MS) for detection of ion speciesthereat, with a readout being available of data D indicative of detectedion species.

As will be understood by a person skilled in the art, the FAIMS filterwith planar surfaces is preferred in embodiments of the presentinvention, but embodiments of the present invention are operable withvarious non-planar parts and surfaces, including filters, detectors,flow paths, electrodes, and the like. The description herein of PFAIMSis by way of illustration and not limitation.

In the embodiments of FIG. 3A, 3B, liquid sample preparation section 10Aincludes electrospray sample ionization source or head 12 having achamber 14 for receipt of liquid sample S. In practice of the invention,the liquid sample S may contain bio-compounds, for example compounds Aand B, in a solvent X. The present invention is engaged to identify oneor more of the compounds in the liquid sample.

In practice of the electrospray device of section 10A, a high voltagepotential 18 is applied by controller 10D to the liquid sample S withinchamber 14 of electrospray head 12. The potential difference between theliquid sample S at electrospray tip 20 and attraction electrode 22,driven by controller 10D, ionizes compounds A, B in solvent X in sampleS in ion region 23. This creates ions 24 and 26, representing compoundsA and B, and solvent molecules 28. In a preferred embodiment, ions andsolvent are driven or drawn along flow path 30 into filter section 10Bbetween the parallel filter electrodes 44, 46 of PFAIMS ion filter 40.

Filtering in the PFAIMS filter device 40 is based on differences in ionmobility, which is influenced by ion size and shape, among other items.This enables separation of ion species based on their characteristics.In one practice of the invention, a high intensity asymmetric waveformradio frequency (RF) signal 48 and a DC compensation signal 50 areapplied to filter electrodes 44, 46 by RF/DC generator circuits withincontroller 10D. The asymmetric field alternates between a high and lowfield strength condition that causes the ions to move in response to thefield according to their mobility. Typically the mobility in the highfield differs from that of the low field. This mobility differenceproduces a net transverse displacement of the ions as they travellongitudinally through the filter between the filter electrodes. In theabsence of a compensating bias signal, these ions would hit one of thefilter electrodes and be neutralized. In the presence of a selectedcompensating bias signal 50 (or other compensation), a particular ionspecies will be returned toward the center of the flow path and willpass through the filter. Therefore, in the presence of the compensatedasymmetric RF signal 48, separation of ions from each other according totheir species can be achieved. Unselected species will hit theelectrodes and be neutralized and species of interest will be passedthrough the filter. The data and system controller 10D regulate thesignals 48, 50 applied to the filter electrodes 44, 46, in order toselect which ion species pass through the filter.

It will be appreciated that it is desirable to isolate ions 24 and 26 tobe able to obtain unambiguous identification of either or both ofcompounds A and B, as can be achieve with the PFAIMS filter 40. ThePFAIMS filter 40 discriminates between ions A and B based on theirmobility, such that in principle only one or the other is presented fordetection at output section 10C according to the compensation applied bycontroller 10D. For example, ions 24 are shown as ions 24′ passed byfilter 40 in FIG. 3A, 3B.

Referring again to FIG. 3A, 3B, the output section 10C includes detector69 with detector electrodes 70, 72. Controller 10D measures the currenton electrodes 70, 72 as an indication of ions passed by filter 40. Theseelectrodes are held at a potential by bias signals 71, 73, fromcontroller 10D. Ions 24′ which passed filter 40 deposit their charge ona detector electrode 70, 72 under control of controller 10D, dependingupon the polarity of the electrode and the control signals 71, 73 on thedetector electrodes. Furthermore, by sweeping the compensation (i.e.,the bias voltage), a complete spectrum of ion species in Sample S can bedetected.

By intelligent control of controller 10D it is possible to selectdifferent operating regimes and as a result it is possible to target thefiltering of ion species of interest. In practice of one embodiment ofthe invention, the asymmetric electric signal 48 is applied inconjunction with compensating bias voltage 50, and the result is thatthe filter passes desired ion species as controlled by electroniccontroller 10D. As well, by sweeping bias voltage 50 over apredetermined voltage range, a complete spectrum of ion species insample S can be achieved.

In another embodiment, the asymmetric electric signal enables passing ofthe desired ion species where the compensation is in the form of varyingthe duty cycle of the asymmetric electric signal, without the need forcompensating bias voltage, again under direction of the control signalssupplied by the electronic controller. By means of these features, theapparatus is also tunable, i.e., it can be tuned to filter ion species,passing only desired selected species to the detector.

A further advantage of the invention is that the filter can passmultiple ion species with similar mobility but different polarity, andthese can be detected simultaneously. If each detector electrode 70, 72is held at a different polarity, then multiple ion species (havingsimilar mobility but different polarity) that pass through the filtercan be detected simultaneously. Detected ions are correlated with theapplied control signals 48, 50 and potential bias signals 71, 73 todetermine the species of detected ion(s) indicated at data D, FIG. 2.

This multi-functionality may be further understood by reference tooutput section 10C, such as in FIG. 3A, where a top electrode 70 is heldat a predetermined voltage at the same polarity as the ions of interestpassed by filter 40 while bottom electrode 72 is held at another level,perhaps at ground. Top electrode 70 deflects ions 24′ downward toelectrode 72 for detection. However, either electrode may detect ionsdepending on the ion charge and polarity and the signal applied to theelectrodes. Thus multiple ion species having similar mobility butdifferent polarity that pass through the filter can be detectedsimultaneously by using top electrode 70 as one detector and bottomelectrode 72 as a second detector, and using two different detectorcircuits in controller 10D, with two different outputs thus emitted.Detector 69 may thus detect simultaneously multiple species passed bythe PFAIMS filter 40, such as a gas sample including sulfur in ahydrocarbon gas background.

The electronics controller 10D supplies the controlling electronicsignals to system 10. A control circuit could be on-board, or off-board,where the PFAIMS device has a control part with at least the leads andcontact pads shown in FIG. 4A that connect to the control circuit 10D.The signals from the controller are applied to the filter electrodes viasuch connections.

In the embodiment of FIG. 4A, a PFAIMS system 10 includes a spectrometerchip 100 having spaced insulated substrates 52, 54, (e.g., Pyrex® glass,ceramic, plastic and the like) with filter electrodes 44, 46 formedthereon (of gold or the like). Substrates 52, 54, define betweenthemselves the drift tube 29 and flow path 30, thus performing a housingfunction. Preferably the substrates are insulating or have surfaces 60,62 for insulated mounting of electrodes. Electrodes 44, 46 form ionfilter 40, with the filter electrodes mounted on these insulatedsurfaces 60, 62 facing each other across the flow path 30.

As shown in FIG. 4A, 4B, 4C, substrates 52, 54 are separated by spacers53, 55, which may be insulating and formed from ceramic, plastic,Teflon® or the like, or may be formed by etching or dicing siliconwafers, or creating an extension of the substrates 52, 54, for example.The thickness of the spacers defines the distance “D” between the facesof substrates 52, 54 carrying electrodes 44, 46. In one embodiment ofFIG. 4A, the silicon spacers can be used as electrodes 53′, 55′ and aconfining voltage is applied by controller 10D to the silicon spacerelectrodes to confine the filtered ions within the center of the flowpath. This confinement can result in more ions striking the detectors,and which in turn improves detection.

In a further alternative embodiment of the invention shown in FIG. 4B,alternative structural electrodes 44 x, 46 x, take the place of thesubstrates 52, 54, and are mounted at and separated by insulatingspacers 53, 55, forming flow path 30 within. At one end of the flowpath, sample preparation section 10A supplies the ions to the filtersection 10B, and at the other end, the filtered ions pass into an outputsection 10C. In the same manner that the substrates serve a structurefunction and form a housing, so too the structural electrodes 44 x, 46 xserve the function of a housing, as well as being electrodes. As withthe substrates, the outer surface of these electrodes may be planar ornot, and may be covered by an insulated surface 61.

In the embodiment of FIG. 4C, shown in side cross-section, theinsulating spacers 53, 55 overlap with the edges 44 f, 46 f of filterelectrodes 44, 46. This ensures that the ions flowing in flow path(i.e., drift tube) 29 are confined to a region of uniform transverseelectric field between the filter electrodes 44, 46, away from theelectrode edges 44 f, 46 f where the non-uniform fringing field “f” ispresent. A further benefit is that all ions are forced to pass betweenthe filter electrodes, and are subjected to that uniform field.

Returning to FIG. 3A, in operation, ions 24, 26 flow into the filter 40.Some ions are neutralized as they collide with filter electrodes 44, 46.These neutralized ions are generally purged by the carrier gas. Purgingcan also be achieved, for example, by heating the flow path 30, such asby applying a current to appropriately configured filter electrodes(e.g., serpentine 44′,46′ shown in FIG. 3D) or to resistive spacerelectrodes. Spacer electrodes 53, 55 of FIG. 4A could be formed withresistive material and therefore could be used as heatable electrodes 53r, 55 r.

Ions 24 are passed to output section 10C of FIG. 3A. Exhaust port 42 isprovided to exhaust the molecules 28 from the passed ions 24. Thisisolation of ions 24 eases the detection function and enables moreaccurate chemical analysis. But even with this precaution, some solventmolecules may remain attached to the ions of interest 24. Therefore, ina preferred embodiment, apparatus is provided to desolvate ions such as24 and 26 prior to their filtering. Desolvation may be achieved byheating. For example, any of electrodes 44, 46, 53 r, 55 r, may have aheater signal applied thereto by controller 10D. In another embodimentincoming gas flow may be heated by heater element 89 as shown in FIG.3B.

It will be appreciated by those skilled in the art that desolvation or“drying” of electrosprayed ions is a critical part of the electrosprayprocess. When the ion is first ejected out the electrospray tip it is inthe form of a droplet with a large amount of solvent coating the ion. Asit travels through the air towards a counter electrode the solventevaporates eventually leaving the desolvated ion which can then beanalyzed. Incomplete desolvation prior to analysis can distort theanalysis. Additionally, a long ion travel distance may be required toallow the ion to sufficiently desolvate, without some other assistance.It will therefore be appreciated that this desolvation is beneficial inpractice of the invention.

In another embodiment of the invention, a symmetric RF-electric field isused to enhance desolvation of ions produced in the electrospray priorto analysis. As shown in FIG. 5A, 5B, a symmetric radio frequency fieldapplied perpendicularly to the carrier gas flow to cause the ionsgenerated in the electrospray process to oscillate symmetrically, and beheated, as they travel down the drift tube so that the ions aredesolvated without net deflection from this signal.

More particularly, the interaction between the ions and the neutralmolecules raises their effective temperature, enhancing theirdesolvation. During their oscillations the ions will impact neutral airmolecules and their internal temperature will increase. The rise in theinternal temperature of the ions enhances the evaporation of the solventand shortens the time to realize a desolvated charged ion. This actionenables desolvation to be done over a relatively short length of thedrift tube. Desolvation results in more accurate detection data, and theabove approach is easily integrated with the PFAIMS filter of theinvention.

The desolvating electric field can be generated by applying a voltagebetween two electrodes configured parallel to each other with a gapbetween them. For example, any of electrode pairs 44, 46 and 53, 55 maybe used for this function, under control of controller 10D. Preferablyseparate desolvation electrodes 77, 79, as shown in FIG. 3B may be usedfor this function.

In a further embodiment of the invention, a micromachined electrosprayhead 80 is mounted on substrate 52, shown schematically in FIGS. 3B and3B1. Electrodes 82, 84, 86, 88 are formed on opposite sides of substrate52 and guide the electrospray ions 24, 26 into ion region 23 of flowpath 30 in drift tube 29. Attraction electrode 22 has a potentialapplied thereto to attract the ions 24, 26 into the ion region 23.Carrier gas flow 90 is set at a desired flow rate to capture ions 24, 26and to carry them to filter 40 for the filtering function alreadydescribed. The gas exhaust 91 includes the carrier gas 90 and carriesaway non-ionized components and neutralized ions.

Potentials applied to electrodes 22, 82, 84, 86, 88, and evendesolvation electrodes 77, 79, can be set and controlled independent ofeach other and of the filter electrodes 44, 46. For example, thisadvantageously enables the attractor electrode 22 to be driven with adifferent signal than any other electrode, such as the adjacent filterelectrode 46. This is particularly facilitated by provision of theinsulated surfaces of the substrates, and the electrode isolation allowsoptimization of ion introduction independent of filter driverequirements.

This configuration also enables the guiding electrodes 82,84, 86, 88 andattractor electrode 22 to be individually operated in a pulsed mode(e.g., switched on and off). In this mode, a select amount of ions canbe introduced into the ion region 23. The time these ions travel, suchas from the orifice to detector 72 for example, can be used in a“time-of-flight” (“TOF”) FAIMS mode of operation. In this mode, the timeof flight is associated with ion species, thus providing additionalinformation for species discrimination. This leads to an improvement incylindrical FAIMS devices.

As will be appreciated by a person skilled in the art of IMS, this TOFis an analog to the time-of-flight practiced in IMS devices, but nowbeing practiced within a FAIMS structure. This new innovation maytherefore provide both IMS and FAIMS detection data in one operatingdevice; the combination of FAIMS and IMS data can yield better detectionresults.

In preferred embodiments, such as shown in FIGS. 3A-3B, 4A-4B, thehousing 64 is formed by substrates 52, 54, with internal flow path 30defined extending from the input part 10A, through the ion filter 10B,to the output part 10C. More particularly, substrates 52, 54 presentwork surfaces 60, 62, which favor formation of electrodes thereat. Thesesurfaces 60, 62 may be curved or planar and preferably insulating (orinsulated), such as when formed using glass or ceramic substrates forexample. This lends itself to mass manufacturing techniques such asMicro-Electro-Mechanical Systems (MEMS) or Multi-Chip Module (MCM) orother processes, with a result of very compact packaging and smallelectrode sizes. As such, the ion filter is preferably defined on theseinsulated surfaces by the facing filter electrodes 44, 46 with the flowpath 30 defined in between, and the insulated surfaces of the substratesin turn then isolating the control signals 48, 50 at the filterelectrodes from detector electrodes 70, 72, for lower noise and improvedperformance. This is unlike the extensive conductive area of the outercylinder of conventional prior art FAIMS devices, such as in U.S. Pat.No. 5,420,424, incorporated herein by reference.

It will be further understood that due to geometrical and physicalconsiderations, the ions in prior art cylindrical designs aredistributed in the drift tube cross-section and therefore only afraction of ions are available in the region R near the mass spec inlet96. In the prior art configuration of a cylindrical FAIMS shown in FIG.6 (see PCT/CA99/00715, incorporated herein by reference), an attempt ismade to overcome this limitation by enabling additional delivery of ionsto the mass spectrometer inlet 96. However neutral sample molecules canalso enter into the mass spectrometer inlet 96 because there is noseparation between the sample ions 24 and neutral molecules, such assolvent molecules 28. This leads to significantly more complex spectrain the mass spectrometer, and degraded resolution.

The present invention overcomes these shortcomings in the configurationof FIG. 3B, for example. In practice of the invention, virtually all ofthe ions 24 entering the detector region 69 are focused into the massspec inlet 96. This results in a dramatic increase in efficiency ofdetection and improved sensitivity of the system, especially compared toa cylindrical FAIMS device where ions are distributed around the entireflow path circumference, not just at the MS inlet.

Furthermore, referring to a new cylindrical design of the presentinvention, shown in FIG. 7A, electrospray tip 20 injects samples viaorifice 31′ in outer electrode 44C into flow channel 30′, underattraction of attractor electrode 22′, and the sample is carried by theflow of gas G toward the filter section 10B′. The attractor electrode isformed adjacent to the inner electrode 46C but electrically isolated byinsulator strips In1, In2. Therefore the attractor electrode can beindependently biased separate from neighboring electrodes, e.g., 46C.This embodiment also combines functional and structural components whilereducing parts count, such as where the inner cylinder components can bemated together via a binding function of the insulating layers In1, In2,for example.

In an alternative embodiment shown in FIG. 7B, an attractor electrode22″ is formed adjacent to outer ring electrode 44C′, insulated therefromby insulator ring In3. The electrospray tip 20 introduces sample S fromthe side into the interior of a ring 46C″, which may be a separateelectrode, or may be an extension of inner electrode 46C′, with thesample under attraction of attractor electrode 22″ and being carried bygas G in flow channel 30″ of filter section 10B″. Again, electrode 22″is isolated from electrode 44C′ by insulator In3, and therefore theelectrodes are independently drivable.

In a further embodiment of the invention shown in FIG. 8, electrosprayassembly 80″, attached to substrate 52, includes electrospray head 12.The ions are carried by guiding electrodes “F” (three in thisembodiment), toward orifice 31 and are attracted into ion region 23 byattraction electrode 22 and guiding electrodes, such as 82, 84 and/or86, 88.

Preferably a separate DC bias “DC” is applied to each guiding electrodeto create a potential gradient which guides the ions towards ion region23. The guiding electrodes can be used for a further function by alsoapplying symmetric RF signals “DS” to enhance desolvation, as earlierdiscussed.

Cleansing gas G is introduced at port P1 to further enhance desolvation.This gas flows opposite to the guided ions in chamber 93 and exhaustsout ports P2, P3. Preferably, this is operated with no pressure gradientacross orifice 31.

In order to improve spray conditions, the separation 20S between the tip20 and the top guiding electrode F1 can be adjusted in practice of theinvention. In one practice, the position of housing 12 a can be adjustedrelative to base B, which in turn adjusts the separation 20S. In analternative, the height of head 12 can be adjusted relative to electrodeF1.

In an alternative embodiment, as shown in FIGS. 9A and 9B, spaced apartguiding electrodes F (FIG. 9A) or F1, F2, F3 (FIG. 9B) are bathed in acurtain gas flow CG. This flow may be unconfined or contained withinhousing H1. The electrospray head 12 is adjustably mounted in mount M1,wherein its angle of delivery can be adjusted relative to the surface ofsubstrate 52. In addition, its height can be adjusted relative to thesubstrate.

Referring again to FIG. 4A, sample reservoir 92 receives a liquid sampleS, which is then ionized and filtered as set forth above. In suchembodiment, a single spectrometer chip 100 integrates both a ionizationsource, such as part of a microfluidic electrospray module 80′, andplanar high field asymmetric waveform ion mobility filter 40. Aninternal detector may also be included, or ions are outputted fordetection. Various micro-fabricated micro-fluidic components may be usedas an ion source, or combinations thereof, including electrospray,nano-electrospray, liquid chromatography, electrophoresis separation.

In another embodiment, the electrospray head 80′ of FIG. 4A may beattached to substrate 52 (preferably through anodic bonding or brazing).Guiding electrodes 82 and 84 are not required in this embodiment.

In the embodiment of FIG. 4D, the microfluidic electrospray module 80′includes sample reservoir 92 feeding a lengthened, serpentine,separation channel 92 a, leading to tip orifice 20′ and then to tip 20.The channel 92 a may be a liquid chromatograph or electrophoreticseparator, or the like, for conditioning or separating constituents inthe sample prior to ionization at the tip 20.

The motivation for such a chip 100, with or without a microfluidicmodule, is to eliminate variability in sample preparation and analysis,this is achieved by reducing human interaction and by providing a devicethat incorporates all key components in a single structure. These chips100 lend themselves to low cost manufacturing and as a result can bedisposable. Using a new chip for each sample analysis eliminates sampleto sample cross-contamination. Additionally, through the reduction inhuman intervention, sample preparation time is reduced. In aconventional arrangement the position of the electrospray tip ormicro-fluidic component, must be re-adjusted each time relative to anyfilter or mass spectrometer inlet. This adds time and cost. With theintegrated micro-fluidics chip/PFAIMS apparatus of the invention, therelative positions of the micro-fluidic components and PFAIMS inlet arefixed. Once analysis is completed the entire chip is simply discardedand a new chip is loaded with a sample to be analyzed and possibly to bemounted on a mass spectrometer. This allows for significantly fasteranalysis times and higher throughput.

In an illustrative embodiment of the invention, shown in FIG. 10A,controller 10D includes several subsystems, including an electrospraycontrol 10D1, a waveform generator (synthesizer) 10D2 cooperating withhigh voltage RF waveform & DC generator 10D3 for applying the RFasymmetric drive signal and DC control bias to filter electrodes 44, 46,and detection electronics 10D4 for detection of ions on the detectorelectrodes. Computer 10D5 collects data and controls the system. In oneembodiment, the RF field is produced in generator 10D3 by asoft-switched semi-resonant circuit that incorporates a flybacktransformer to rapidly generate the high voltage pulses. The circuitprovides a peak-to-peak RF voltage of at least 1400 volts at a frequencyof around 100 KHz-4 MHz with a duty cycle of about 10-70%. Sample RFwaveforms for driving the filter electrodes are shown in FIG. 10B,although variations thereof are also within practice of the invention.

Preferably the chip 100 is inserted into a chip receiver assembly 220.Assembly 220 includes a socket 222 for receipt of the chip. The socketis electrically connected to the controller 10D. A preferred embodimentof chip receiver 220 serves a further function of coupling the chemicalsensor system 10 to a mass spectrometer MS 98, as shown in FIG. 11B.Chip receiver assembly 220 is affixed to the face 224 of the massspectrometer, such that outlet orifice 99 of system 10 is aligned viaorifice 99 x with the MS orifice inlet 96, whereby ions 24′ are directedinto the MS for detection and analysis.

Detection of ions 24 passing through filter 40 may be made as describedabove in conjunction with the detector electrodes 70, 72 of FIG. 3A. Analternative embodiment is shown in FIG. 3B where electrode 70 is nowused as a deflector electrode to deflect ions 24′ toward intake 96 ofmass spectrometer 98. The ions are guided or focused by focusingelectrodes 72 a, 72 b and pass through an orifice 99 in substrate 54′and through plenum gas chamber 101 via a mounting adapter 102. Providinga low flow rate plenum gas into chamber 101 prevents neutralized sampleions or solvent molecules from entering the mass spectrometer intake 96.Ions that are focused into the mass spectrometer intake are thendetected according to standard mass spectrometer procedures. It will beappreciated that the plenum chamber 101 is not shown in FIG. 11B,although it may be beneficially used in this embodiment.

An assembly of the invention can be easily mounted right up against themass spectrometer inlet 96 (with or without a plenum chamber), as shownin FIGS. 3B, 11B and 12A-12B, for example. The deflector electrode (sidemounting FIG. 3B or 12A-12B) allows almost 100% of ions to be deflectedinto the mass spectrometer.

This high efficiency is in contrast with the prior art cylindricaldesign in FIG. 12C-12D, mounted to inlet 96 of the mass spectrometer,where only a small fraction of the total ions in the drift tube areaffected by the electric field which propels them into inlet 96 andresulting in only a fraction of the available ions being detected in theprior art.

It will now be appreciated that in practice of the invention, chemicalanalysis can be performed using any of several ion detectors. In theembodiments of FIG. 3A and 4A, the detector is entirely internal to theassembly 10. In the embodiment of FIG. 3B, assembly 10 is intimatelymated via adapter 102 to the mass spectrometer 98 as a detector. In theembodiment of FIG. 3B, if the current on focusing electrodes 72 a, 72 bis monitored, then additional detector information is available forprocessing the detection information of mass spectrometer 98. Evenwithout focusing electrodes 72 a, 72 b, a FAIMS spectra of the inventioncan be reconstructed by monitoring the total ion current in the massspectrometer.

Alternative embodiments of the invention are shown in FIGS. 13A, 13Bwhere the electrospray tip 20 has been inserted within ion region 23,either from above through orifice 31 in upper substrate 52′ (FIG. 13A)or from the side (FIG. 13B). Attractor electrodes 104, 106 attract andguide the ions in the flow path 30 as they travel in gas flow 90 towardfilter electrodes 44, 46. In FIG. 13A, droplets from the electrospraytip 20 collect in reservoir 54 a, which also may be provided with adrain hole 54 b.

It is desirable to concentrate ions after they pass through the ionfilter and before entering output section 10C. This improves the signalto noise ratio at the detector and improves sensitivity. An ion trap orion well can collect ions in this manner, concentrating them and thendelivering the concentrated ions at once to the output section. Neutralsare not collected in the ion trap and are continuously being removed bythe gas flow from the ion trap T.

An ion trap can be applied to many embodiments of the invention, such asin FIG. 3A,B,C, for example. An illustrative embodiment is shown in FIG.13A, where an ion trap T is formed with several appropriately biasedelectrode pair. In one example, for positive ions, the electrodes arebiased such that a potential minimum is formed in the region ofelectrode pair 76 b and potentials on electrode pairs 76 a and 76 c arehigher. Ions are allowed to accumulate in the trap, and after a desiredamount of time resulting in collection of a desired number of ions, thetrap can be opened by adjusting the voltages applied to electrodes 76 a,76 b and 76 c. When the trap is opened, the trapped ions 24′ flow intothe output section 10C.

In the embodiments discussed above, ion filter 40 includes spacedelectrodes 44, 46 which are driven by the RF and DC generator 10D3 asions are propelled by gas flow 90 in drift tube 29. In the embodiment ofFIG. 14A, 14B, a longitudinal electric field driven embodiment of theinvention, a novel method of conveying the ions in the drift tube 29 isshown.

In the embodiments of FIG. 14A, 14B, the ions are propelled toward theoutput section 10C using a longitudinal electric field generated byelectrodes 110 and 112. These embodiments feature a simplified gas flowstructure in a very compact design, and gas flow is even optional.

In one embodiment, ions actually travel in an opposite direction to gasflow 122, and are propelled by electric field vector 120. This gas flowopposite to the ion travel direction enhances the desolvation of thesample ions. It also maintains a clean ion filter 40 free of neutralsample molecules. This consequently decreases the level of ion clusterformation resulting in more accurate detection of ion species.Furthermore the counter gas flow clears out and reduces memory effectsof previous samples in ionization region 23. This embodiment can includeintegrated electrospray tip 20 inserted within ion region 23 from above,or side mounted, as are shown.

In the longitudinal electric field driven embodiments of FIG. 14A, 14B,ions 24, 26 are conveyed without gas flow 122 but rather by action of alongitudinal electric field produced by sets of cooperating electrodes110, 112 along with a longitudinal RF & DC generator 10D3′. As anexample of the operation of the PFAIMS in a particular electrode biasscheme, several or all of the electrode pairs 10 a-h, 112 a-h have thesame RF voltage applied, while the DC potentials are stepped so that alongitudinal potential gradient is formed to drive the ions towards thedetector. This embodiment can operate without a gas flow or optionallycan include an exhaust gas flow 122 which exhausts neutrals and solventmolecules out exhaust port 124.

In one example, electrodes 110, 112 a might have 10vdc applied theretoand electrodes 110 h, 112 h then might have 100vdc applied. Now negativeions in region 10A are attracted by electrode pair 110 a-112 a andfurther attracted by pair 110 h, 112 h, and their momentum then carriesthem into detector region 10C if passed by the filter.

The RF and compensation may be applied to various of the electrodes 110a-h, 112 a-h, and will operate in the manner set forth above.

In another embodiment of FIG. 14A the electrospray tip can be externalto ionization region 23 (not shown) above orifice 31 where electrode 112j serves as the attraction electrode. In the longitudinal electric fielddriven embodiment of FIG. 14B, the ion filter includes spaced resistivelayers 144,146 insulated from electrodes 134, 136, by insulating medium140, 142, for example, a low temperature oxide material. Preferably thesubstrates are insulating. Resistive layers 144, 146 are preferably aceramic material deposited on insulating layers 140,142. Terminalelectrode pairs 150, 152, 154, 156 make contact with a resistive layerand enable a voltage drop across each resistive layer to generate thelongitudinal electric field vector 120. Electrodes 150 and 154 arebiased according to application, for example they may be at 1000 voltswhile electrodes 152 and 156 maybe at zero volts.

When the embodiment of FIG. 14B is implemented in a cylindrical design,then the electrodes 150 and 154 form a ring electrode, and electrodes152 and 156 form a ring electrode, and resistive layers 144, 146 form acylinder.

The present invention can also perform time of flight ion mobilityspectrometry functions. For example, in the embodiment of FIG. 14A,electrodes 104, 106 are pulsed to draw a sample from tip 20 that isionized, starting the time cycle. Electrodes 110 a-h, 112 a-h are biasedrelative to their neighbors so that the ions are driven by the generatedlongitudinal electric field gradient towards output section 10C. Acounter gas flow 122 can be applied to sweep sample neutrals away. Acombination of these electrodes can be used to form the ion trap Tdescribed above (see FIG. 13A).

In the split gas flow embodiment of FIG. 15A, the electrospray needle 12is inserted through substrate 52 and into ion region 23, however, it maybe mounted externally to the drift tube such as in FIG. 3A. The ion flowgenerator in this design includes a plurality of segmented electrodes160, 162 on opposite sides of flow path 30 to create longitudinalelectric field E. In the preferred embodiment, one or more discreteelectrodes 160′, 162′ are located downstream of gas inlet 170 to extendlongitudinal electric field E beyond the split flow of gas, and therebyensuring that ions flow into filter 40 as carried by drift gas flowstream 172.

In the embodiment of FIG. 15B, mass spectrometer 98 is directly coupledto the end of the drift tube 30. An advantage of this design is that theion filter 40 is kept free of sample neutrals by virtue of the split gasflow. This prevents clustering of neutral sample molecules with ions,and this results in higher detection accuracy. A venting device 103 forventing of neutrals N keeps neutrals out of the MS intake.

A baffle 174 may be placed as shown to regulate the velocity of wastegas flow stream 176 relative to the velocity of drift gas flow stream172. Typically, drift gas flow stream 172 is at a higher velocity thanwaste gas flow stream 176. Other means for creating a waste gas flowstream of a velocity different than the drift gas flow stream, however,are within the scope of this invention.

In the embodiments of FIG. 15A, 15B, various sample preparation sectionscan be used, whether simple a port to draw in ambient air samples, orelectrospray, gas chromatograph, liquid chromatograph, or the like.Regardless of what is used, the split gas embodiment shown can preventclustering and allows better identification of ion species.

Generally the sample ions tend to be found in monomer or cluster states.The relationship between the amount of monomer and cluster ions for agiven ion species is dependent of the concentration of sample and theparticular experimental conditions (e.g., moisture, temperature, flowrate, intensity of RF-electric field). Both the monomer and clusterstates provide useful information for chemical identification. It willbe useful to investigate the same sample separately in a condition whichpromotes clustering, and in an environment that promotes the formationof only the monomer ions. A planar two channel PFAIMS of an embodimentsuch as shown in FIG. 16 can be used to achieve this.

In the dual channel embodiment of FIG. 16, a first channel “I” is shownfor receipt of ions 24, and molecules 28 in a drift gas flow 190 in ionregion 194. Also included are PFAIMS filter electrodes 44, 46 anddetector electrodes 70, 72.

To interrogate the sample ions in the monomer state, the ions areextracted from the flow stream (by action of an electric field betweenelectrodes 198 and 200) and they flow up into upper chamber “II”. Theneutral molecules 28, typically solvent, continue to flow throughchannel “I” and exit at drift gas exhaust 192. The potential differencebetween the electrospray tip 20 and the attraction electrode 191accelerates the ions into the ion region 194 through orifice 196 insubstrate 56. A second gas flow 202 prevents the sample neutrals fromentering chamber “II” and carries ions 24 to PFAIMS filter 40(electrodes 44, 46 in Chamber II), and the passed ions are thendetected, such as with detector electrodes 70, 72 as in FIG. 3A or witha mass spectrometer as in FIG. 3B. The second gas flow 202 exhausts asflow 204. When the deflection and attractor electrodes 198, 200 are notenergized, then the sample ions can be observed in the cluster state inchamber “I” by the local detector electrodes 72 and 70. By alternativelyenergizing and not energizing electrodes 198 and 200 significantly moreinformation can be obtained to better identify the chemical sample.

FIG. 17 shows a homologous series of Ketone samples obtained in onepractice of the invention, ranging from Butanone to Decanone. From thefigure it is evident that for the same chemical species the cluster ions(top plot) require very different compensation signals compared to themonomer ions (bottom plot). So by observing the difference in peakposition of the monomer and cluster peak the level of identification ofthe chemical compound can be significantly increased. For example, forButanone the peak position in the monomer state occurs close to −9 voltswhile the cluster peak is around zero. For Decanone for example, themonomer peak is close to zero while the cluster peak is at around +4volts.

The motivation for the embodiment shown in FIG. 18 is the same as thatof embodiment 16. In this system switching between a monomer state andcluster state operating condition is achieved by control of a curtaingas flow 190 a and 192 a. With the curtain gas applied, sample neutrals28 are prevented from entering channel “II” and ions in the monomerstate can be investigated. Curtain gases 190 a and 192 a may flow in thesame direction and exhaust at orifice 196 for example. Meanwhile the gasflows in channel “II” remain in the same configuration as the system inFIG. 16 Guiding electrodes 206 and 208 are included to guide the ionsinto channel “II”. Attraction electrode 200 is also used to attract ionsinto channel “II”. When the curtain gas is turned off, ions in thecluster state may be observed since sample neutrals and sample ions maynow be drawn into channel “II” using a pump 204 a. Gas flows 202 and 204may also be used. The output section may be connected to a massspectrometer.

In application of the present invention, the high field asymmetric ionmobility filtering technique uses high frequency high voltage waveforms.The fields are applied perpendicular to ion transport, favoring a planarconfiguration. This preferred planar configuration allows drift tubes tobe fabricated inexpensively with small dimensions, preferably bymicromachining. Also, electronics can be miniaturized, and totalestimated power can be as low as 4 Watts (unheated) or lower, a levelthat is suitable for field instrumentation.

We have described novel apparatus that combines electrospray andfiltering components. We further disclose micromachinedPFAIMS-electrospray interface chips. The PFAIMS-electrospray interfacechips offer unique benefits compared to all prior bio-molecule-filteringmethods for electrospray mass spectrometry. At the same time thisapproach can be used in conjunction with many in-liquid separationtechniques such as capillary electrophoresis.

In practice of an embodiment of the invention, tributylamine waselectrosprayed into the PFAIMS filter and detector. Resulting spectraare shown in FIG. 19 for the amine in solvent and for the solvent eluentalone. There is virtually no response for the eluent alone, andsignificant response to the amine. This demonstrates practical value andfunction of the invention.

The present invention provides improved chemical analysis in a compactand low cost package. The present invention overcomes cost, size orperformance limitations of prior art TOF-IMS and FAIMS devices, in novelmethod and apparatus for chemical species discrimination based on ionmobility in a compact, fieldable package. As a result a novel planar,high field asymmetric ion mobility spectrometer device can be intimatelycoupled with a electrospray tip to achieve a new class of chemicalsensor, i.e., either as a standalone device or coupled to an MS. Afieldable, integrated, PFAIMS chemical sensor can be provided that canrapidly produce accurate, real-time or near real-time, in-situ,orthogonal data for identification of a wide range of chemicalcompounds. These sensors have the further ability to render simultaneousdetection of a broad range of species, and have the capability ofsimultaneous detection of both positive and negative ions in a sample.Still further surprising is that this can be achieved in acost-effective, compact, volume-manufacturable package that can operatein the field with low power requirements and yet it is able to generateorthogonal data that can fully identify various a detected species.

Another advantage of the PFAIMS design over prior art cylindricaldesigns is the ability of the PFAIMS to filter and act on all types ofions with different alpha α dependencies on electric field strength (seebackground section for more detail on alpha α). This fact allowssignificant reduction in the complexity of performing measurements inunknown complex sample mixtures.

It will be appreciated by a person skilled in the art that in the priorart cylindrical design shown in FIG. 12C-D, the radial electric fielddistribution is non-uniform. Meanwhile, in practice of the presentinvention, such as the PFAIMS shown in FIG. 3A,B, the field distributionbetween the ion filter electrodes (neglecting fringing fields) in thePFAIMS design is uniform and the field is uniform.

It has been found that the time for separation of ions in the planarFAIMS design is significantly less (˜10 times) than in the prior artcylindrical FAIMS design when reaching conditions for ion focusing.

However, embodiments of the present invention may be practiced in methodand apparatus using cylindrical, planar and other configurations andstill remain within the spirit and scope of the present invention.Examples of applications for this invention include use in biologicaland chemical sensors, and the like. Various modifications of thespecific embodiments set forth above are also within the spirit andscope of the invention. The examples disclosed herein are shown by wayof illustration and not by way of limitation. The scope of these andother embodiments is limited only as set forth in the following claims.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. An ion mobility based analyzer comprising: a liquid samplepreparation section including: a liquid chromatograph for conditioningthe liquid sample, and an electrospray assembly for ionizing the liquidsample, an ion mobility based filter for receiving the ionized sampleand passing through selected ion species while the ions are flowingthrough a time-varying field, and an output section for detecting thepassed ion species.
 2. The analyzer of claim 1 comprising an insulatingspacer extending along a portion of a flow path of the ions through thefilter.
 3. The analyzer of claim 2, wherein the ion mobility basedfilter includes a pair of filter electrodes, the filter electrodes beinginsulated from each other, at least in part, by the insulating spacer.4. The analyzer of claim 2, wherein the insulating spacer extendssubstantially from an inlet to an outlet of the filter.
 5. The analyzerof claim 1, wherein the ion mobility based filter includes a pair offilter electrodes, the filter electrodes being separated by at least oneinsulating spacer, the spacer extending along a flow path of the ionsthrough the filter.
 6. The analyzer of claim 1, wherein the ion mobilitybased filter includes a pair of filter electrodes, the filter electrodesbeing separated by a pair of insulating spacers.
 7. The analyzer ofclaim 6, wherein the insulating spacers oppose each other.
 8. Theanalyzer of claim 1, wherein the electrospray assembly includes anelectrospray ionization source.
 9. The analyzer of claim 1, wherein theoutput section includes a mass spectrometer.
 10. The analyzer of claim1, wherein the output section includes a ion trap.
 11. The analyzer ofclaim 1, wherein the output section includes an ion trap incommunication with a mass spectrometer.
 12. A method for analyzing asample comprising: preparing a liquid sample including: conditioning theliquid sample using a liquid chromatograph, and ionizing the liquidsample using an electrospray assembly, receiving the ionized sample andpassing through selected ion species of the ionized sample while theions are flowing through a time-varying field of an ion mobility basedfilter, and detecting the passed ion species.
 13. The method of claim 12comprising extending an insulating spacer along a portion of a flow pathof the ions through the ion mobility based filter.
 14. The method ofclaim 13, wherein the ion mobility based filter includes a pair offilter electrodes, the filter electrodes being insulated from eachother, at least in part, by the insulating spacer.
 15. The method ofclaim 13 comprising extending the insulating spacer substantially froman inlet to an outlet of the ion mobility based filter.
 16. The methodof claim 12, wherein the ion mobility based filter includes a pair offilter electrodes, the filter electrodes being separated by at least oneinsulating spacer, the spacer extending along a flow path of the ionsthrough the filter.
 17. The method of claim 12, wherein the ion mobilitybased filter includes a pair of filter electrodes, the filter electrodesbeing separated by a pair of insulating spacers.
 18. The method of claim17, wherein the insulating spacers oppose each other.
 19. The method ofclaim 12, wherein the electrospray assembly includes an electrosprayionization source.
 20. The method of claim 12 comprising detecting usinga mass spectrometer.
 21. The method of claim 12 comprising trapping thepassed ion species.
 22. The method of claim 21 comprising detectingusing a mass spectrometer.
 23. An ion mobility based analyzercomprising: an ion filter for receiving an ionized sample and passingthrough selected ion species while the ions are flowing through atime-varying field, and an insulating spacer extending along a portionof the flow path of the ions through the filter.
 24. The analyzer ofclaim 23, wherein the ion filter includes a pair of filter electrodes,the filter electrodes being insulated from each other, at least in part,by the insulating spacer.
 25. The analyzer of claim 24, wherein at leastone filter electrode is adjacent to the insulating spacer.
 26. Theanalyzer of claim 24, wherein at least one filter electrode is mountedto the insulating spacer.
 27. The analyzer of claim 24, wherein theinsulating spacer extends substantially from an inlet to an outlet ofthe filter.
 28. The analyzer of claim 23, wherein the ion filterincludes a pair of filter electrodes, the filter electrodes beingseparated by the insulating spacer, the insulating spacer extendingalong a flow path of the ions through the filter.
 29. The analyzer ofclaim 23, wherein the time-varying field includes an asymmetric field.30. The analyzer of claim 23, wherein the ion filter includes acompensation field.
 31. The analyzer of claim 23, wherein the insulatingspacer is includes at least one of silicon, ceramic, plastic, andTeflon®.
 32. The analyzer of claim 23, wherein the insulating spacer isformed from etching or dicing a silicon wafer.
 33. A method foranalyzing a sample comprising: receiving an ionized sample and passingthrough selected ion species while the ions are flowing through atime-varying field of an ion mobility based filter, and extending aninsulating spacer along a portion of the flow path of the ions throughthe ion mobility based filter.
 34. The method of claim 33, wherein theion mobility based filter includes a pair of filter electrodes,comprising insulating the filter electrodes from each other, at least inpart, using the insulating spacer.
 35. The method of claim 34 comprisingpositioning at least one filter electrode adjacent to the insulatingspacer.
 36. The method of claim 34 comprising mounting at least onefilter electrode to the insulating spacer.
 37. The method of claim 34comprising extending the insulating spacer substantially from an inletto an outlet of the filter.
 38. The method of claim 33, wherein the ionmobility based filter includes a pair of filter electrodes, comprisingseparating the filter electrodes using the insulating spacer andextending the insulating spacer along a flow path of the ions throughthe ion mobility based filter.
 39. The method of claim 33, wherein thetime-varying field includes an asymmetric field.
 40. The method of claim33, wherein the ion mobility based filter includes a compensation field.41. The method of claim 33, wherein the insulating spacer is includes atleast one of silicon, ceramic, plastic, and Teflon®.
 42. The method ofclaim 33, wherein the insulating spacer is formed from etching or dicinga silicon wafer.